1. Field of the Invention.
The present invention relates, in general, to a system, method and apparatus for demodulating a transmitted signal contaminated with multi-path distortions, and, more particularly, to a system, method, and apparatus for transmitting and receiving data using a conventional quadrature amplitude modulated (QAM) or vestigial side band (VSB) modulated signal.
2. Relevant Background.
A broadcast, cable, or other communication channel through which a television signal is transmitted adds various types of distortions to the transmitted signal. One type of distortion is caused by reflections of the transmitted signal such that the received signal is composed of a directly received signal and a plurality of delayed signals that reach the receiver through several reflecting routes. The delayed signals are commonly referred to as "multi-path interference", "ghosts", or "echoes".
A typical ghost canceling circuit includes an analog-to-digital converter for converting a received signal such as a television signal (usually an analog video signal such as an NTSC, PAL, SECAM, or digital signals such as 8-VSB, 64-or 256-QAM signals, and the like) into a binary signal. The digitized signal is processed through an adaptive equalizer in the time domain or frequency domain to cancel the echoes.
There are two main steps to the ghost canceling process. First, the communication channel is characterized to determine a frequency response of the channel at the receiver. The frequency response essentially includes all of the echoes caused by the transmission channel. Once the channel characteristics are determined, filters are used to multiply the inverse of the channel frequency response characteristics with the received signal so as to cancel the ghosts. One such ghost cancellation system is described in U.S. Pat. No. 5,568,202 issued to Koo on Oct. 22, 1996.
There is increasing interest in transmission of "non-picture" data (i.e., closed-caption text or video-enhancing data), simultaneously with the picture data normally viewed on a television screen. Typically, this data is transmitted as one or more horizontal lines during the vertical blanking interval (VBI) of the television signal. There are also attempts to encode data in other non-visible portions of the television signal. While echoes caused by the communication channel are undesirable for visual data, they are often tolerable because the viewer can learn to ignore the ghosts. In contrast, echoes are intolerable in data transmission because even minor distortions raise the bit error rate (BER) of the communications channel to unusable levels. Although error correction coding can mitigate some of the errors created by echoes, this coding limits the data transmission rate and cannot practically compensate for severe echoes. A need exists for a demodulation system that cancels ghosts, and a demodulation method and apparatus with improved ability to remove ghosts.
Previous ghost cancellation systems such as described by Koo, cited above, use adaptive equalizers that are useful for correcting linear distortions affecting transmission channels. Impairments such as in-home reflections on cable wiring, broadcast echoes or ghosts, as well as diplex filter group delay and amplitude non-flatness are examples of linear impairments that can be corrected. This de-ghosting process characterizes the transmission channel by determining an impulse response of the transmission channel. A "ghost cancellation reference" (GCR) signal is placed in one line of the television signal's VBI and provides a known impulse stimulus to the transmission channel that can be detected at the receiver. The impulse response is used to calculate tap coefficients for a finite impulse response filter (FIR) or infinite impulses response filter (IIR) used in the adaptive equalizer.
These systems work well for relatively short duration echoes. The FIR circuits have a limited number of taps for cost and size reasons, and hence are effective only for canceling echoes of relatively short delay. These systems can only remove echoes when the energy from the main or direct path signal and the energy from all of the echoes from the main signal are contained within the same block (i.e., can be processed simultaneously by the FIR filter). Any echo energy that is not contained within the block cannot be canceled by a FIR filter, so an IIR filter must be used. The quality of the solution is strongly deteriorated if there are an insufficient number of taps relative to the delay and strength of the echo. If the echo is long and strong, the solution is poor. On the other hand, IIR filters cannot guarantee stability and so may cause undesirable oscillation and generally cannot be made to have a linear phase response.
The impulse response may be transformed into the frequency domain where the echo appears as a ripple in the frequency response. The reciprocal of the delay of the echo is the period of the frequency response ripple, and the amplitude of the echo is represented by the amplitude of the frequency response ripple. This dual view of the channel's appearance with an echo is possible because of the discrete Fourier transform (DFT) operation used to transform a signal between the time domain and the frequency domain.
In one data transmission technology called "orthogonal frequency division multiplexing" (OFDM) a "guard interval" can be used to overcome the effect of echoes shorter than the guard interval. In OFDM, a digital signal (such as a television signal) is transformed using an inverse discrete Fourier transform (IDFT) before it is applied to the transmission channel. At the receiver, the signal is transformed by a forward DFT to recover the transmitted signal. A guard interval is inserted before each block of the IDFT signal before transmission. The guard interval usually consists of a cyclic extension of the IDFT output blocks. Provided that the guard interval length (i.e., time duration) is larger than echoes in the channel's impulse response, the cyclic prefix makes the linear convolution of the channel appear as a circular convolution that can be more accurately transformed by the discrete Fourier transform process at the receiver. An example of guard-interval protected OFDM data encoded into a conventional television signal is shown in U.S. Pat. No. 5,371,548 issued to Williams on Dec. 6, 1994 and assigned to the Assignee of the present invention. The guard interval technique has not been applied to conventional vestigial side band (VSB) transmissions such as 8-VSB and National Television Standards Committee (NTSC) signals or to QAM signals such as 64-QAM.